DO NOT MERGE: WebAssembly standalone support by jerbob92 · Pull Request #103 · bblanchon/pdfium-binaries

jerbob92 · 2023-03-20T11:26:30Z

@bblanchon I promised to come back to you when I finished my WebAssembly implementation, so here it is! It's not ready to merge this in, since it will break support for the web build, but it is possible to use this as a base to create 2 WASM builds: web and standalone.

This contains the following changes:

A patch for PDFium to add support for the callbacks, this was necessary because the runtime that I use (Wazero) doesn't support adding functions to the function table dynamically, so it basically adds these function pointers during compile time and that calls back into the host function of the runtime. This does mean that you can't just start loading the WebAssembly file on any runtime, since you do have to implement these host methods, but the same could be said for WASI and the host calls that the web builds add. We might want to add some documentation on these methods though?
A patch for Emscripten to implement the WASI methods in the standalone build to make file operations work
An update to the latest Emscripten (you probably want to include this for the web build too)
A switch between debug/non-debug builds that also includes optimizations (you might want to partially include this for the web build too)
A change in the build step that also includes other methods that should be exposed (you might want to partially include this for the web build too)

The thing that I'm not really happy with is how the Emscripten patch is applied, but I couldn't get it to work in another way, since emsdk/upstream/emscripten is not a git repo.

… a dummy version in the runtime

…ported callbacks

…first char

bblanchon · 2023-03-23T12:57:53Z

Hi @jerbob92,

Thank you very much for sharing this!

I didn't expect that we'd have to patch emscriptem and add a new features PDF.
I think this is out of the scope of this repository: the goal here is to provide a pre-built PDFium, not extend PDFium.
Do you think these changes could be integrated upstream?

I'll try to cherry-pick the changes you recommend right away.
I originally downgraded emscriptem in f09df4a to work around the error "Uncaught RuntimeError: unreachable"; hopefully, this bug is gone.

Best regards,
Benoit

jerbob92 · 2023-03-23T13:40:53Z

I think Emscripten is not going to accept the patch in this state because it only implements what PDFium uses, nothing else.

And regarding extending PDFium: I don't think there is a way to work around this, to use function pointers in WebAssembly, the pointer needs to exist during compile time or you need to add the pointer to a method during runtime, which many runtimes don't support, that's why I had to implement various callbacks in the PDFium patch and then implement the callback logic on the runtime side. So I wouldn't really see it as extending PDFium, this is just a way to make all functionality work on in WebAssembly.

I originally downgraded emscriptem in f09df4a to work around the error "Uncaught RuntimeError: unreachable"; hopefully, this bug is gone.

I think this was related to the memory allocator that has since been disabled.

jerbob92 · 2023-06-28T19:43:11Z

@bblanchon small warning, free and malloc don't seem to export by default anymore in the latest Emscripten, I had to add them to EXPORTED_FUNCTIONS myself. Just in case you want to upgrade Emscripten in the future. The build works fine but people using the Webassembly build will need access to free and malloc, although I'm not sure if that's also the case for JS users.

jerbob92 · 2025-07-21T13:07:12Z

    cd emsdk
-    ./emsdk install ${EMSDK_VERSION:-latest}
-    ./emsdk activate ${EMSDK_VERSION:-latest}
+    ./emsdk install ${EMSDK_VERSION:-4.0.11}


I have changed this back to a static version, to make sure that emscripten.patch applies.

jerbob92 · 2025-07-21T13:07:35Z

  echo "pdf_is_standalone = true"
  echo "pdf_use_partition_alloc = false"
-  echo "target_cpu = \"$TARGET_CPU\""
+  if [ "$TARGET_CPU" == "wasm-standalone" ]; then


This felt like the best solution to me right now to prevent a lot of patches, let me know what you think.

jerbob92 · 2025-07-21T13:09:41Z

@bblanchon I have updated the MR to make it possible to have this next to the normal wasm build, this introduces the CPU target wasm-standalone, I'm not sure if it's useful for anyone else, but it would be nice to have this inside the main repo.

jerbob92 · 2025-07-21T13:10:18Z

+    if [ "$CPU" == "wasm-standalone" ]; then
+      pushd "$PWD/upstream/emscripten"
+      patch -p1 --forward < "$PATCHES/wasm/emscripten.patch" || true
+      rm -Rf cache


This is to make sure emscripten recreates "prebuilt" libraries.

bblanchon · 2025-07-25T07:42:15Z

Hi @jerbob92,

I'm sorry for not getting back to you sooner.
I only found the time to take a quick glance at this PR, and what struck me immediately is the huge patch of emscriptem.
I don't think it's the role of the present project to maintain such a big change in emscripten; I know I would be unable to maintain the code.
Can't this be moved upstream?

Best regards,
Benoit

jerbob92 · 2025-07-25T10:25:05Z

Thanks for checking @bblanchon! The emscripten patch is actually from emscripten-core/emscripten#18285
It's not merged in yet, but work is being done to merge this upstream in smaller parts. It doesn't cost me a lot of time to maintain, I mostly just merge in the latest tag into my implement-more-syscalls-for-standalone branch and that's it.

bblanchon · 2025-07-27T07:58:27Z

I'd be more comfortable if we could wait for this PR to be merged.
Do you think it has a chance of being merged anytime soon?

jerbob92 · 2025-07-28T06:54:38Z

I'd be more comfortable if we could wait for this PR to be merged. Do you think it has a chance of being merged anytime soon?

It's been moving forward really slowly since 2022, so probably not soon.

bblanchon and others added 16 commits September 19, 2022 12:49

!!! Remove all other builds

c247675

WASM: add STANDALONE_WASM=1

1d2ad0d

Build with symbols, use WasmFS

7120a4d

Don't build with the partition allocator, don't use WasmFS

d0ff2ef

Add data availability

0af40b8

Merge master

337544a

Update emsdk

afa0802

Add emscripten patch

cdb8a8f

Remove timer support for standalone so that we dont have to implement…

28787cf

… a dummy version in the runtime

Add data avail structs and callbacks

2a487fd

Fix some exports not being included, rename patch, add time and unsup…

6238d73

…ported callbacks

Add formfill support

7510a03

Add progressive rendering support, fix flag mapping in open syscall

8b59993

Add support for debug build, add build optimization

dc211b0

Fix build command

7c62c7c

Fix up Emscripten path_open implementation, we need to strip off the …

6ee69d6

…first char

bblanchon pushed a commit that referenced this pull request Mar 23, 2023

WASM: Update emsdk (#103)

c2ca13f

bblanchon pushed a commit that referenced this pull request Mar 23, 2023

WASM: Export FPDF*, FSDK*, FORM*, IFSDK* (#103)

deb6f6f

bblanchon pushed a commit that referenced this pull request Mar 23, 2023

WASM: Add support for debug build, add build optimization (#103)

e556343

jerbob92 added 5 commits June 28, 2023 16:59

Update emscripten patch

6e94250

Update emscripten patch

b9c1542

Merge in latest master, update emscripten

303e05b

Fix build

9d19d9a

Add free and malloc to exported functions

56ef8f3

jerbob92 added 2 commits July 25, 2023 22:02

Update to emsdk 3.1.44, clear cache when downloading and patching emsdk

36eca91

Merge remote-tracking branch 'origin/master' into wasm-standalone

33bdec6

sungaila mentioned this pull request Aug 29, 2023

Unable to statically link PDFium WASM module #122

Closed

jerbob92 added 3 commits July 19, 2024 13:56

Update emscripten patch

f89de70

Merge remote-tracking branch 'origin/master' into wasm-standalone

9fd68b1

Update emscripten + patch

5be9610

bblanchon force-pushed the master branch from c6abc88 to f2df3ee Compare June 11, 2025 12:44

jerbob92 added 5 commits July 21, 2025 13:19

Merge master

6bc9d23

Update emscripten patch

78d2f18

Add wasm-standalone support

855e06d

Use static version for emscripten to make sure patch applies

1d5d235

Remove extra changes

f0ad51f

jerbob92 commented Jul 21, 2025

View reviewed changes

jerbob92 mentioned this pull request Aug 4, 2025

Reproducible wasm builds klippa-app/go-pdfium#248

Closed

0xbe7a mentioned this pull request Aug 7, 2025

Add bindings to support Wasm WASI. ajrcarey/pdfium-render#159

Closed

bblanchon force-pushed the master branch 6 times, most recently from 5f9317d to 8af2e14 Compare September 8, 2025 14:54

jerbob92 added 5 commits April 10, 2026 14:58

Merge in changes from original

24d737c

Fix build (at least on OSX)

e9f6bf0

Upgrade to Emscripten 5.0.5

ec41777

Merge in master

059b3fb

Upgrade to Emscripten 6.0.0

40e55d8

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DO NOT MERGE: WebAssembly standalone support#103

DO NOT MERGE: WebAssembly standalone support#103
jerbob92 wants to merge 51 commits into
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